In a conventional electronic key system for vehicles, an electronically operable hand-held device (portable device) carried by a vehicle user transmits its identification information (ID code) in response to a request transmitted from an electronically operable in-vehicle device mounted in a vehicle. The in-vehicle device compares the ID code received from the portable device and an ID code stored in its memory. The in-vehicle device permits predetermined operations, such as locking or unlocking of doors or starting of an engine of a vehicle, if the compared ID codes agree.
JP 2002-322841 proposes that a portable device detachably accommodates therein a mechanical key (key plate) as an emergency key for use in case of inoperability of the portable device because of battery rundown. JP 2000-64680 proposes that an in-vehicle device allows registration of additional portable devices by storing each ID code of such additional portable devices.
It is necessary that the in-vehicle device confirms that the registration of each additional portable device is requested by an authorized user from the standpoint of security. The in-vehicle device may use, as a key registration verification ID code, the same ID code as assigned to the original portable device for the normal door lock/unlock control or engine operation to confirm that the registration of the additional portable device is requested by the authorized user.
In this case, vehicle security may not be ensured. For instance, if a driver who rents a vehicle successfully registers an additional portable device without authorization, the driver can steal the vehicle by the additional portable device even after the vehicle is returned.
To overcome this problem, the key registration verification ID code may be stored in a different body such as a card, which is separate from the portable device and not normally carried. The different body however is sometimes inconvenient. For instance, it is likely to be lost or unavailable at the time of requesting the additional registration, because it is not usually carried.